When it comes to your body, you essentially have two systems

On one hand you have a rigid system which is immediately necessary for life. On the other hand you have the plastic system, which is much more dynamic in nature and aids in maintaining the rigid systems.

Due to the fact that rigid systems are immediately necessary for life, they have a small range where disruption can occur. Therefore, any disruptions in the rigid system cause amplified disruptions in the dynamic system.

Because the plastic system supports the rigid, it can handle much larger perturbations (as demonstrated by my squiggly lines).

Example:

Human ATP stores are rigid; without ATP we basically die. Therefore, ATP doesn’t have much wiggle room to be perturbed. However, our bodies have multiple ways of producing ATP from the various energy systems (dynamic).

On the other hand, our mind only knows how to go from point A to point B while our subconscious control (the autonomic nervous system) has to find a way to make it happen.

Let’s say you decide to workout for 4 hours (not recommended). Your body will find a way to do it, but your dynamic systems will be greatly taxed. You will burn through your creatine phosphate stores, muscle and liver glycogen, immediate fat reserves, and eventually you will start drawing energy from proteins derived from other muscles. The further you dig into your “deep” plastic reserves, the more you will have to payback and recovery will be longer. Demands might have been great enough that in order to maintain the rigid systems, your plastic systems take a beating causing a number of cascading events.

This sometimes happens with runners when volumes are increased to drastically. The plastic system gets thrashed and the athlete doesn’t recover like they should. Chronic loading of overly long distances doesn’t allow the plastic systems to function adequately and boom, you have a stress fracture.

The functions of the plastic reserves are never conscious, otherwise we would go insane trying to actively run our own body. However, proper recovery across the board (sleep, stress reduction, nutrition) can give our plastic system the tools needed.